1. Field of the Invention
The invention relates to a structure of memory unit and, more particularly, to a memory with multi-bits and fabrication method thereof.
2. Description of the Related Art
Memory devices for nonvolatile storage of information are currently in widespread use, in a myriad of applications. A few examples of nonvolatile semiconductor memory include read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM) and flash EEPROM.
In many circuit designs it is desirable to have a nonvolatile memory device that can be erased and reprogrammed in-circuit without the need to remove the device.
High-density nonvolatile memory devices have been receiving much attention for application in many fields. One of the most important factors is the low cost of the reduced size of each memory cell.
Only one set of data is programmed to or erased from the flash memory at a time.
In FIG. 1a, high voltage is applied to a control gate 105, and electrons enter a floating gate 103 from a source 101a in the silicon substrate 101 through the gate oxide layer 102. Thus, the flash memory is programmed.
In FIG. 1b, low voltage or no voltage is applied to the control gate 105 and high voltage is applied to the drain 101b in the silicon substrate 101, thus electrons return to the source 101a from the floating gate 103 through the gate oxide layer 102. Thus, the flash memory is erased.
Eitan discloses a two-bits non-volatile electrically erasable and programmable semiconductor memory cell in U.S. Pat. No. 6,011,725, in which two sets of data are stored in the non-volatile electrically erasable and programmable semiconductor memory cell.